The present invention relates to a system and method for burners, and more particularly, to a system and method for mass flow control and for characterizing the fuel/air ratio for a burner used on dryers for hot mix asphalt plants and the like such that explosive and very lean conditions are avoided and continued operation of the dryer takes place in the event of malfunction of the mass flow measuring equipment.
A hot mix asphalt plant is typically used to produce asphalt pavement and includes a feed bin system for proportioning and conveying aggregate mixes into the plant, a rotating dryer drum to heat and dry the aggregate, a burner/combustion heating system to provide heat to aggregate in the rotating dryer drum, a flue gas exhaust system for conveying and filtering the exhaust gas from the dryer/heating drum, a mixing device to mix the hot aggregate with the asphalt cement, and a conveyor to move the hot mix asphalt to the user's trucks.
Generally speaking, the burner has separate air and fuel valves and associated control motors such as found in the "EcoStars" system made by Hauck Manufacturing Co., the assignee of the present application. These fuel valves can be of the butterfly type for natural gas control or, in the case of liquid fuel, an oil control valve such as the Hauck "Microvalvel". The combustion air control valve in the Hauck "EcoStar" can be a rotating multi-vane inlet fan damper or a multiple blade fan outlet damper. These different valve have different "percentage of opening versus percentage of flow" curves, and may also have different speed control valve drive motors such as Models EA-57, EA-71 or EA-73 made by Barber-Colman.
Dryers or aggregate dryers for use on hot mix asphalt plants frequently used either a mechanically or electronically linked fuel and air valves to maintain correct fuel/air ratios. These known fuel/air rationing systems typically did not, however, have the capability of varying the fuel/air ratio at in-between points in the travel of the valves without affecting the fuel/air ratios at the other points.
More recent electronic systems which have programmable relationships between the fuel and air valves and thereby solve the problem of in-between point adjustable fuel/air ratios without affecting the other point relationships. The ability to characterize the fuel to the air is necessary because different types of valves frequently have different flow-versus-position relationships which substantially affect the fuel and air ratios. For example, if the fuel valve has 50% of its flow at 40% opening and the air valve has 50% of its flow at 60% opening, then the proper fuel/air positioning setting is 60% opening on the air with 40% opening on the fuel valve. such electronic control device for use in the dryer burner combustion industry is described in U.S. Pat. No. 5,190,454. That known system uses a programmable logic controller or PLC for setting the characterizing relationships based on flue gas analysis.
Another electronic control is offered by North American Mfg. Co. as Models 8080/8081 controller. However, the known mechanical or electronic control approaches have not recognized the need to limit the danger of rich operation which is a matter of particular concern on a dryer or aggregate dryer.
In particular, previous electronic controls used either the fuel valve or the air valve as the temperature-controller valve and the other of the valves as the follower. In the case of the fuel valve always being controlled during a temperature increase the fuel will first increase first and then the air valve will follow. This can lead to a rich condition, particularly, as is often the case, if the fuel drive motor drive is faster than the air valve drive motor. This momentary rich condition creates a dangerous situation in a dryer because the dryer operates below typical auto-ignition temperatures for most fuels and additional air for completing the combustion is frequently available downstream of the combustion section. A combustible mixture would be created downstream of the combustion zone in the exhaust ductwork and could explode if an ignition source were present. This momentary rich condition also creates unwanted emissions of unburned hydrocarbons or carbon monoxide.
It is, therefore, an object of the present invention to prevent as much as possible the occurrence of undesirable rich and lean operating conditions.
It is another object of the present invention to provide flue gas recirculation for further NO.sub.x emission reduction.
It is yet another object of the present invention to measure and maintain proper fuel/air ratios even with malfunctioning of flow measuring components.
The foregoing objects have been achieved in accordance with the present invention by providing and electronic programmable linkage system using, for example, an integrated or stand along PLC which assures that the air and fuel valves do not move in such a way to create a momentary but dangerous rich condition as the system moves from one firing rate to another.
In particular, the present invention reduces the risk of this rich condition by incorporating logic to change the valve being driven by the temperature controller to maintain a leaner condition during modulation. In order to maintain a lean condition, the air valve is controlled by the temperature controller when a burner output increase is desired, and the fuel valve is controlled by the temperature controller when a decrease of burner output is desired. Consequently, the air "leads" the fuel on increasing demand, and the fuel "leads" the air for a decrease in burner output.
One advantage of the present invention is that an unwanted rich condition is prevented to the extent possible. In accordance with another aspect of the present invention, if one control valve drive motor is faster than the other control valve drive motor, the present invention prevents the valve driven by the slower motor from lagging too far behind the other valve when moving to a new position. This valve-lagging condition could momentarily create undesirable flue gas emissions of lean combustion products of combustion. To reduce this lean condition as much as possible, the present invention uses logic which stops the valve being driven by the temperature controller if the follower valve lags too far behind.
The amount of acceptable lag is adjustable within the skill in this art and is set in the PLC. For example, the amount of acceptable lag might be 5% of the range of valve motor travel. That is, if the air valve drive motor was being driven due to a desire for an increase in burner output and it had travelled to 50% of its travel and the fuel valve motor was at 44% of its travel, then the air valve would be interrupted from going to 51% until the fuel valve had arrived at 45% of its travel. Of course, this example is merely illustrative and is not intended to limit the principles of the present invention.
Another, more sophisticated embodiment of the present invention is describable as a mass flow system with electronic programmable valve positioning of the foregoing type as a back-up. The mass flow system also includes a system for controlling fuel and air ratio of burners by measuring the fuel flow, the air flow, and, where appropriate, flue gas flow. With measurement of each of the flows, the system can calculate the correct fuel/air ratio and, if appropriate, the flue gas recirculation ratio. The mass flow system according to this embodiment also includes the same apparatus and method described above for preventing rich operation when a change in firing rate is required. The only difference is that instead of comparing the valve motor positions as the electronic programmable linkage does, the PLC compares the measured fuel an air flows to the calculated desired fuel air ratio.
The above-described electronic programmable linkage used mass flow system of this second embodiment can be used in the event of a problem or required maintenance with the flow measuring equipment to allow for continued operation of the dyer without a serious interruption. This switch over to the programmable electronic positioning can occur automatically whenever the mass flow logic detects a problem with the flow sensing transmitters (such as zero output during operation) or it can be enabled below at any demand rate such as, for example, 10% of flow demand. For example, at 9% of flow demand, the system would automatically use programmable electronic valve positioning. This can be particularly advantageous because many flow transmitters, such as differential pressure-to-current transmitters, are limited in accurate flow sensing to a 6:1 turndown range or 16.7% of flow demand.